1. Field of the Invention
The present invention relates to integrated circuit manufacturing, and more particularly to a method of making insulated-gate field-effect transistors.
2. Description of Related Art
An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel.
Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, and the polysilicon is anisotropically etched to provide the gate. Thereafter, the gate provides an implant mask during the implantation of source and drain regions, and the implanted dopants are driven-in and activated using a high-temperature anneal that would otherwise melt the aluminum.
An important parameter in IGFETs is the threshold voltage (V.sub.T), which is the minimum gate voltage required to induce the channel. In general, the positive gate voltage of an N-channel device must be larger than some threshold voltage before a conducting channel is induced, and the negative gate voltage of a P-channel device must be more negative than some threshold voltage to induce the required positive charge (mobile holes) in the channel. There are, however, exceptions to this general rule. For example, depletion-mode devices already have a conductive channel with zero gate voltage, and therefore are normally on. With N-channel depletion-mode devices a negative gate voltage is required to turn the devices off, and with P-channel depletion-mode devices a positive gate voltage is required to turn the devices off.
Complementary metal-oxide semiconductor (CMOS) circuits include N-channel and P-channel devices. CMOS manufacturing typically includes growing a single gate oxide layer for the N-channel and P-channel devices, then forming the gates for the N-channel and P-channel devices by depositing a blanket layer of polysilicon over the substrate, forming a photoresist layer over the polysilicon layer, etching portions of the polysilicon layer beneath openings in the photoresist layer, and stripping the photoresist layer. Thereafter, typically arsenic and/or phosphorus is used to dope the source and drain for the N-channel device, and boron is used to dope the source and drain for the P-channel device.
A problem encountered in P-channel devices with polysilicon gates containing a high concentration of boron is that when a thin gate oxide is used, poor threshold voltage control may arise due to unwanted boron penetration into the gate oxide, or further, into the underlying channel region. It is reported that boron will penetrate gate oxides that are less than 125 angstroms thick during a 900.degree. C. 30-minute post-implant anneal in nitrogen. It has also been found that the presence of fluorine in the gate oxide worsens the boron penetration problem. Such fluorine can be introduced into the gate oxide if boron difluoride (BF.sub.2) is the implant species. In some instances, the boron penetration may severely disruption the threshold voltage.
Alternatively, if the gate oxide or the gate is sufficiently thick to suppress boron penetration, the increased dimensions may also disrupt the threshold voltage, which is particularly undesirable for N-channel devices where the gates are usually free of boron.
Accordingly, a need exists for a method of making a transistor, particularly in CMOS processes, which improves threshold voltage control.